SI-7003 Partest redirects stderr to log file

Some scalac output is on stderr, and it's useful to see that
in the log file, especially for debugging.

Adds a line filter for logs, specified as "filter: pattern"
in the test source.

Backslashes are made forward only when detected as paths.

Test alignments:

Deprecations which do not pertain to the system under test
are corrected in the obvious way.

When testing deprecated API, suppress warnings by deprecating
the Test object.

Check files are updated with useful true warnings, instead of
running under -nowarn.

Language feature imports as required, instead of running under -language.

Language feature not required, such as casual use of postfix.

Heed useful warning.

Ignore broken warnings. (Rarely, -nowarn.)

Inliner warnings pop up under -optimise only, so for now, just
filter them out where they occur.

Debug output from the test required an update.
SI-7198 Par-Test uses filters files

Partest will also read files/filters and files/kind/filters for
filter expressions (one per line, trimmed, leading #comments)
which are taken as regexes.

A test/files/filters is provided which attempts to quell HotSpot
warnings; the test for this commit requires it.

The elided lines can be revealed using the lemon juice of verbosity:

apm@mara:~/projects/snytt/test$ ./partest --verbose --show-diff files/run/t7198.scala
[snip]
>>>>> Transcripts from failed tests >>>>>

> partest files/run/t7198.scala
% scalac t7198.scala
[snip]
% filtering t7198-run.log
--Over the moon
--Java HotSpot(TM) 64-Bit Server VM warning: Failed to reserve shared memory (errno = 28).

The filtering operation is part of the transcript, which is printed on failure.

No attempt is made to be clever about not slurping the filters file a thousand times.

Previous literal patterns had to be updated because there's parens in them thar strings.

Future feature: pattern aliases, define once globally and invoke in test filters.
Merge pull request #2591 from som-snytt/topic/partest-inches

SI-7003 Partest redirects stderr to log file